FIG. 1 shows a schematic diagram of elements from a BICM (Bit Interleaved Coding and Modulation) module within a communications receiver. This receiver may, for example, be a Digital Terrestrial Television (DTT) receiver, such as a DVB-T2 receiver. The demapper 102 receives cells 104 and uses noise variance estimates 106 in order to output soft information 108 (which may also be referred to as soft estimates), such as Log-Likelihood Ratios (LLRs). This soft information 108 is passed to the decoder 110. In some examples, soft information is fed back to the demapper from the decoder (as indicated by dotted arrow 112).
Conventionally, the noise variance is estimated using the pilots within the broadcast signal. Each pilot cell is modulated with reference information that is known to the receiver and the locations of pilots are typically specified in the transmission standard. Assuming the pilots are represented as xs and the channel is represented as h, then the received symbol at a pilot bearing sub-carrier can be represented as:ys=h·xs+ns.
This signal is passed through a noise-reduction filter, which is a low pass filter characterised by an “equivalent noise bandwidth” ENBW, which can simply be described as the percentage of the Nyquist bandwidth that the filter passband occupies. The filter is selected such that the channel component is not attenuated by the filter.
After applying the noise-reduction filter to the received signal ys, the output of the filter can be represented as ŷs=h·xs+ns2, where ŷs represents the filtered signal and ns2 represents the filtered noise samples. The channel estimate can be obtained as:
      h    ^    =                              y          ^                s                    x        s              =          h      +                                    n                          s              ⁢                                                          ⁢              2                                            x            s                          .            
In order to estimate the noise samples outside the equivalent noise bandwidth of the filter, the following operation is done:
      y    s    =                              h          ·                      x            s                          +                  n          s                    ⇒              n        s              =                                        y            s                    -                      h            ·                          x              s                                      ⇒                  n          ^                    =                                    y            s                    -                                    h              ^                        ·                          x              s                                      =                                            h              ·                              x                s                                      +                          n              s                        -                                          (                                  h                  +                                                            n                                              s                        ⁢                                                                                                  ⁢                        2                                                                                    x                      s                                                                      )                            ·                              x                s                                              =                                                                      h                  ·                                      x                    s                                                  +                                  n                  s                                -                                  h                  ·                                      x                    s                                                  +                                  n                                      s                    ⁢                                                                                  ⁢                    2                                                              ⇒                              n                ^                                      =                                          n                s                            -                              n                                  s                  ⁢                                                                          ⁢                  2                                                                        
The above equation results in the complex noise estimate per pilot. The resultant noise is not the effective noise estimate. It has to be scaled by the effective noise bandwidth of the noise-reduction filter in order to get the effective noise per pilot. The noise variance per pilot is then evaluated as the power of the noise estimate per pilot and these per-pilot values are stored. Then the noise variance estimate is interpolated for the whole symbol.
The embodiments described below are not limited to implementations which solve any or all of the disadvantages of known methods of estimating noise variance for an OFDM (Orthogonal Frequency Division Multiplex) signal.